All electronic and electric devices require a power supply which provides the required voltage and current for the electronic or electric devices to be operated. The power supply is typically connected to mains voltage. As long as the power supply is indeed coupled to the mains voltage and the mains voltage is sufficiently high, the operation of the electric or electronic device will be ensured. However, if the mains voltage fails, for example due to short mains interruptions in the electrical grid, then the electronic and electric devices will not be supplied with sufficient energy and cannot operate as required.
US 2002/0171295 A1 discloses a semiconductor integrated circuit with two power supply lines. A switch is provided for each power supply line to select one of the power lines. A power supervisory circuit monitors the voltage on a main power source. If the main power supply is operating, then the semiconductor integrated circuit will be supplied with power from the main power source. However, if the power supply from the main power source is interrupted, a backup power source will supply the required power for the semiconductor integrated circuit.
Typical electronic equipment does not have a backup power source and can therefore not operate under mains power down conditions. Nevertheless, also normal electronic equipment should survive short mains interruptions. The energy needed during these short mains interruptions will be typically provided by a large electrolytic capacitor.
FIG. 1 shows a block diagram of an electric or electronic device according to the prior art. The device comprises an AC/DC conversion unit ADC which receives the mains voltage MV as input and which is coupled at its output to a DC-bus voltage capacitor C0. The capacitor C0 is furthermore coupled to a subsequent system SS which supplies at its output a load L. The AC/DC conversion unit ADC e.g. performs an AC/DC conversion. To the output of the AC/DC conversion stage a bus capacitor Co is connected which forms a DC bus that supplies an output voltage Vo to the subsequent systems. The subsequent system SS can for example be implemented as a DC/DC converter. The load L can be any electric or electronic device. The capacitor C0 is typically a large electrolytic capacitor providing a nearly constant output voltage V0 for the subsequent system SS. The capacitor C0 has to fulfill several requirements. It should be large enough to avoid a high ripple voltage. Furthermore, the capacitor must be selected appropriately to ensure the hold-up requirement of the system, i.e. a power level demanded by the load L must be available during a short failure of the mains voltage. To achieve this, the output voltage V0 must remain above a specified threshold voltage for a specified period of time. Accordingly, a capacitor with a large value is provided to ensure a long hold-up time. The capacitance value of the capacitor C0 must be large enough to provide power for the subsequent system which may comprise several further subsystems. However, large valued capacitors will typically have a large size even if electrolytic capacitors are used. In addition, if a large capacitor is selected when the capacitor is charged for the first time, a high in-rush current will be present, which may damage other parts of the system or disturb other electronic equipment in the neighborhood.